1. Technical Field
The present invention relates generally to control circuits and in particular to ballast circuits used to drive gas discharge lamps, such as fluorescent lamps.
2. Background Art
In one prior art approach, a gas discharge lamp is connected into a resonant circuit. Control of the current flowing through the lamp is accomplished by varying the frequency of the AC signal driving the circuit. Maximum current is delivered to the lamp where the frequency of the pulse signal equals the resonant frequency of the circuit. As the frequency of the pulse signal diverges away from the resonant frequency, there is an attendant dropoff of current flowing through the lamp.
The chief advantage of this approach is its relative simplicity. However, this approach suffers from known disadvantages. Gas discharge lamps exhibit a nonlinear behavior, in which the voltage across the lamp increases as the lamp current decreases. This affects the range of frequency variation required for a given desired range of dimming. Further, the frequency of the pulse signal used to drive the circuit cannot fall below a critical threshold frequency, i.e., the loaded resonant frequency. Below this threshold, the circuit begins to oscillate in a "capacitive" mode, leading to destruction of circuit components. In addition, the overall efficiency of the circuit may be compromised, as circuit components do not exhibit optimal performance throughout the range of frequencies that may be needed for control.